Objective To investigate the effectiveness and safety of oral olive oil combined with polyethylene glycol electrolyte (PEG) on bowel preparation before colonoscopy for chronic constipation patients. Methods A randomized, single-blind, prospective study was conducted on 180 patients with chronic constipation, who underwent colonoscopy at Tangdu Hospital of the Fourth Military Medical University from November 2017 to May 2018. Patients were randomly divided into three groups. Patients in group A took 60 mL olive oil and a piece of crystal sugar at 7:30 pm the day before colonoscopy, followed by 1 500 mL PEG at 8:00 pm before the test, and 1 500 mL PEG at 5:00 am the day of colonoscopy. Patients in group B took 60 mL olive oil and a piece of crystal sugar after administration of PEG at 8:00 pm the day before colonoscopy, followed by 1 500 mL PEG at 5:00 am the day of colonoscopy. The group C was given 1 500 mL PEG at 8:00 pm the day before colonoscopy, and another 1 500 mL PEG at 5:00 am the day of colonoscopy. We recorded the time of first defecation after taking medication, the number of defecation before sleep, the total number of defecation, the score of Boston bowel preparation scale (BBPS) of the right, middle and left colon, and the adverse reactions, and compared the data among the three groups. The measurement data was compared using the analysis of variance. After the difference was found, the LSD-t test was used to compare between the two groups. The enumeration data was compared using the Pearson χ2 test. Results One patient in the group B terminated colonoscopy due to unable to continue cooperation during the examination. Group B and C both excluded one patient because of a large mass found in the descending colon of patients. Finally, there were 60 cases in the group A, 58 in the group B, and 59 in the group C. There was no statistical difference between the three groups in the general resource (P>0.05). The time of first defecation after taking medication for the group A, B and C was (2.25±2.32) h, (2.43±2.39) h and (3.36±2.79) h respectively, with statistical difference (F=3.36, P=0.037). The time of first defecation was longer in the group C than that of the group A and B (P=0.016 and P=0.046, respectively). The number of defecation before sleep for the group A, B and C was 3.47±2.09, 3.24±1.76 and 2.49±1.58 respectively, with statistical difference (F=4.65, P=0.011). The number of defecation before sleep was lesser in the group C than that of the group A and B (P=0.004, P=0.027, respectively). The total number of defecation for the group A, B and C was 7.20±2.67, 6.81±2.31 and 5.64±2.22 respectively. The difference among the three groups was statistically significant (F=6.68, P=0.002). For the group A and B, the total number of defecation was both more than that of the group C (P=0.001, P=0.010). There were no statistical differences among the three groups in the BBPS score of the left and middle colon and the total BBPS score (all P>0.05). The BBPS score of the right colon for the group A, B and C was 2.03±0.82, 1.95±0.87 and 1.53±0.80 respectively, with statistical difference (F=6.38, P=0.002), and was lower in the group C than that of the group A and B (P=0.001, P=0.006, respectively). Adverse reactions after taking medication including nausea, vomiting, abdominal pain, and bloating were respectively reported in 7, 3, 0 and 3 cases in the group A, 5, 3, 0 and 6 in the group B, and 4, 2, 1 and 4 in the group C, and there was no statistical difference among the three groups (χ2=4.35, P=0.824). Conclusion Administration of olive oil compared with PEG can improve the cleanness of right colon for chronic constipation patients, shorten the time of first defecation after taking medication, and increase the number of defecation before sleep and the total number of defecation during bowel preparation. Taking olive oil before or after PEG at the night before colonoscopy has no significantly effect on bowel preparation and adverse reactions.